1. Field of the Invention
The present invention relates generally to a bending machine. In particular, the invention concerns an improvement of a bending tool for bending under pressure a blank material such as an elongated metal sheet into a section or a channellike member having a V-like, L-like, Z-like or similar cross-section.
2. Description of the Prior Art
Numerous and various types of metallic channel members of L-like, V-like or Z-like cross-section are widely used as brackets, reinforcing members and so forth for various structures machines, equipments or the like. The channel members of these cross-sections are generally manufactured by using hydraulic press bending machines such as one shown in FIG. 1 and commercially available from Promecam Company in France, or Amada Company in Japan, for example. For having a better understanding of advantageous features of the invention, a typical one of the hitherto known hydraulic press bending machine will be described in some detail.
Referring to FIG. 2 which shows in an enlarged fragmental perspective view a portion of the hydraulic press bending machine designated generally by reference letter P in FIG. 1, a bending tool comprises a male die or punch 2 mounted on a main body 1 of the bending machine P and a female die 3 having a cavity of V-like cross-section and mounted on a table 4, both dies being disposed in a vertical alignment with each other. Usually, the table 4 is moved vertically upward so that the male die or punch 2 is engaded in the V-like cavity of the female die 3, whereby a metal sheet 6 interposed between the punch 2 and the V-like female die 3 is bent into a corresponding section or channel member. Typical processes for manufacturing a L-like or Z-like channel member or section with the aid of the bending machine P are illustrated in FIGS. 3 to 5.
In the first place, reference is made to FIG. 3 which illustrates a working process of bending a flat metal bar into a L-like section. The bending tool comprises a male die or punch 2 fixedly mounted on the main body 1 of the bending machine P by means of bolts 5 or the like and a female die 3 having a cavity of V-like cross-section and mounted fixedly on the table 4 of the bending machine. The punch 2 and the female die 4 are disposed in vertical alignment and in opposition with each other. The sheet material 6 to be bent is interposed between the punch 2 and the female die 4, whereby the sheet or flat material 6 is bent into a L-like section under pressure produced by moving one of the punch 2 and the female die 3 to the other, i.e. through relative displacement of the male and female dies toward each other in the vertical direction. As can be seen from FIG. 3, the lower end portion of the male die or punch 2 is formed with a pointed bending edge having a bending angle of about 90.degree. and a perpendicular bisector passing through the pointed edge. On the other hand, the female die 3 has the V-like cavity which is defined by inclined opposite side faces intersecting each other at about 90.degree. and has a perpendicular bisector extending through the intersection. The V-like punch 2 and the V-like female die 3 are geometrically and positionally so aligned to each other that the pointed bending edge portion of the V-like bending punch 2 will snugly fit in the cavity of the female die 3 in a manner complementary to each other. Accordingly, when the V-like bending punch 2 is pressed down into the V-like cavity of the female die 3 with the sheet material 6 being interposed therebetween, the latter is forcively fitted into the V-like cavity of the female die 3 under the pressing and bending action of the V-like punch (i.e. male die) 2 and undergoes plastic deformation to be shaped into a L-like section. In this way, the L-like section or channel member can be fabricated.
Referring to FIG. 4 which illustrates a typical one of the conventional processes for bending a sheet metal into a Z-like section, a punch 7 having a working lower end of a corresponding sectional form is fixedly secured to the main body 1 of the bending machine P by means of bolts 5 or the like, while a die 8 having a sectional form complementary to that of the punch 7 is secured to the table 4 in a manner similar to the structure illustrated in FIG. 3. By moving one of the punch 7 and the die 8 to the other or both in opposition to each other in the vertically aligned direction, the metal sheet 6 disposed between the punch 7 and the die 8 is bent into a Z-like section. It will be seen that when the punch 7 is caused to engage in the die 8, the bending edge portion 9 of the punch 7 is complementarily and snugly fitted in a cavity or groove 10 of the die 8 while V-like edge 11 thereof is caused to engage in a groove 12 formed in the lower end face of the punch 7 in a complementary manner. Accordingly, by selecting the height of the bending edges as well as the depth of the cavities of the punch 7 and the die 8 at proper values, a metal sheet 6 having a predetermined thickness t and disposed on the die 8 can be bent into a Z-like section in conformance with the profiles of the bending members 7 and 8, whereby a Z-like section member having a predetermined bend width h can be manufactured.
The hitherto known press bending machine described above has however encountered some difficulties which will be briefly discussed below.
The first problem lies in the fact that the punch and the counterpart die are substantially unexchangeably combined in a set for manufacturing a L-like or Z-like section of a predetermined dimension or size. As the consequence, in order to manufacture the L-like or Z-like sections or channel members of different dimensions, a corresponding number of the bending tools, i.e. the sets of punches and dies have to be previously provided. Each time when the dimension or size of the section to be formed is changed, the bending tool has to be replaced by other one of the corresponding dimension. This holds true even in the case where a section to be formed differs in dimension only a little from the preceeding one. It goes without saying that preparation of a large number of the bending tools in view of numerous types of the L-like or Z-like sections or channel members involves necessarily a great expenditure and that replacement of the punches and the counterpart dies in dependence on the dimensions or sizes of the products is a time consuming and troublesome procedure. Such problem becomes more serious in the bending work for manufacturing the Z-like section. For example, the combination of the bending punch 7 and the die 8 for producing the Z-like section illustrated in FIG. 3 can be used effectively only for the product having the bend width h of a predetermined value. For producing the Z-like sections of different widths h, a corresponding number of the tools have to be previously prepared. If there is available no appropriate tool for a specific dimension of the section, a new bending tool (i.e. a new combination of punch and die) has to be first manufactured.
The second problem can be seen in the fact that when the bend width h is required to be very small as compared with the sheet thickness t, as is illustrated in FIG. 5, the bending operation can no more be effected with a satisfactory accuracy with the hitherto known tool. More specifically, when a metal sheet material 6 is to be bent into a Z-like section with the bend width h of a very small value by means of the appropriate set of the punch 7 and the die 8, a portion of the sheet 6 which extends outwardly from the portion sandwiched between the punch 7 and the die 8 tends to be bent downwardly about a point A instead of extending straight along a phantom line e, as illustrated in an enlarged scale in FIG. 5a. On the other hand, the portion of the sheet 6 sandwiched between the bending members is squeezed and tends to be decreased slightly in the thickness, resulting in the formation of a slight bulge in the vicinity of the point A. This phenomenon may be explained by the fact that the sheet material located immediately below the bending edge 9 of the punch 7 or located immediately above the bending edge 11 of the die 8 (see FIGS. 4 and 5) can not reach the bottom of the counterpart cavity due to the small dimension of the bend width h, whereby the material being deformed tends to be displaced outwardly, to bring about a so-called shear droop. In this way, it has been very difficult to bend a metal sheet into the Z-like section of a small dimension with an acceptable accuracy by means of the hitherto known bending tool assembly.